Preparation and biological evaluation of 64Cu-CB-TE2A-sst2-ANT, a somatostatin antagonist for PET imaging of somatostatin receptor-positive tumors.
Recently, the somatostatin receptor subtype 2 (SSTR2) selective antagonist sst2-ANT was determined to have a high affinity for SSTR2. Additionally, 111In-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-sst2-ANT showed high uptake in an SSTR2-transfected, tumor-bearing mouse model and suggested that radiolabeled SSTR2 antagonists may be superior to agonists for imaging SSTR2-positive tumors. This report describes the synthesis and evaluation of 64Cu-CB-4,11-bis(carboxymethyl)-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane-sst2-ANT (64Cu-CB-TE2A-sst2-ANT) as a PET radiopharmaceutical for the in vivo imaging of SSTR2-positive tumors.


METHODS
Receptor-binding studies were performed to determine the dissociation constant of the radiopharmaceutical 64Cu-CB-TE2A-sst2-ANT using AR42J rat pancreatic tumor cell membranes. The internalization of 64Cu-CB-TE2A-sst2-ANT was compared with that of the 64Cu-labeled agonist 64Cu-CB-TE2A-tyrosine3-octreotate (64Cu-CB-TE2A-Y3-TATE) in AR42J cells. Both radiopharmaceuticals were also compared in vivo through biodistribution studies using healthy rats bearing AR42J tumors, and small-animal PET/CT of 64Cu-CB-TE2A-sst2-ANT was performed.


RESULTS
The dissociation constant value for the radiopharmaceutical was determined to be 26 +/- 2.4 nM, and the maximum number of binding sites was 23,000 fmol/mg. 64Cu-CB-TE2A-sst2-ANT showed significantly less internalization than did 64Cu-CB-TE2A-Y3-TATE at time points from 15 min to 4 h. Biodistribution studies revealed that the clearance of 64Cu-CB-TE2A-sst2-ANT from the blood was rapid, whereas the clearance of 64Cu-CB-TE2A-sst2-ANT from the liver and kidneys was more modest at all time points. Tumor-to-blood and tumor-to-muscle ratios were determined to be better for 64Cu-CB-TE2A-sst2-ANT than those for 64Cu-CB-TE2A-Y3-TATE at the later time points, although liver and kidney uptake was significantly higher. Small-animal imaging using 64Cu-CB-TE2A-sst2-ANT revealed excellent tumor-to-background contrast at 4 h after injection, and standardized uptake values remained high even after 24 h.


CONCLUSIONS
The PET radiopharmaceutical 64Cu-CB-TE2A-sst2-ANT is an attractive agent, worthy of future study as a PET radiopharmaceutical for the imaging of somatostatin receptor-positive tumors.
Wadas
Thaddeus J
TJ
Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, Missouri, USA.
Eiblmaier
Martin
M
Zheleznyak
Alexander
A
Sherman
Christopher D
CD
Ferdani
Riccardo
R
Liang
Kexian
K
Achilefu
Samuel
S
Anderson
Carolyn J
CJ
D000595:Amino Acid Sequence; D000818:Animals; D045744:Cell Line, Tumor; D008297:Male; D009369:Neoplasms / Q000000981:diagnostic imaging* / Q000378:metabolism*; D009942:Organometallic Compounds / Q000737:chemistry*; D010455:Peptides / Q000737:chemistry* / Q000378:metabolism / Q000493:pharmacokinetics; D049268:Positron-Emission Tomography; D011854:Radiochemistry; D019275:Radiopharmaceuticals / Q000737:chemistry; D051381:Rats; D017481:Receptors, Somatostatin / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D014018:Tissue Distribution
C483895:(4,11-bis(carboxymethyl)-1,4,8,11-tetraazabicyclo(6.6.2)hexadecane)copper(II); D009942:Organometallic Compounds; D010455:Peptides; D019275:Radiopharmaceuticals; D017481:Receptors, Somatostatin; C093733:somatostatin receptor 2